📅 OpEx Dynamics

OpEx — monthly options expiration — falls on the third Friday of every month. A large fraction of open interest rolls off that day, which mechanically reshapes the gamma board. The two effects worth knowing are pinning into OpEx and the post-OpEx drift.

Pinning into OpEx

When a strike has dominant open interest and dealers are long gamma there, their hedging is mean-reverting. As price drifts above the strike they sell; as it drifts below they buy. The result is often a tight range around the strike into Friday's close.

💡 : Look at the GEX heatmap on Thursday afternoon of OpEx week. A single dominant strike near spot with heavy call-and-put gamma is the pinning signature.

The post-OpEx drift

After Friday's close, the bulk of expiring open interest is gone. The board re-weights toward later expirations, and because dealers often held net long gamma in the expiring tenor, aggregate gamma usually drops. Less mean-reverting hedging = bigger realized moves the following week — this is the widely observed post-OpEx weakness effect.

How to read it on GammaBaba

Open the GEX Multi-Panel during OpEx week to compare the expiring tenor against the next two.
Use the GEX Flow panel over Friday → Monday to quantify how much aggregate gamma rolled off.
On Monday, check whether the new King Strike is meaningfully far from spot — that extension often sets the week's tone.

💡 OpEx effects are stronger on single stocks than indices, because indices now have heavy weekly and 0DTE flow that dilutes the monthly cycle.

🔑 Key Takeaways

OpEx = 3rd Friday — major open-interest roll-off.
Pinning: dominant long-gamma strike + spot near it = tight Friday range.
Post-OpEx drift: gamma drops, realized vol typically expands.
Single-stock OpEx effects > index OpEx effects (due to 0DTE/weeklies).

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